1. Field of the Invention
The present invention relates to the fabrication of an improved heterojunction bipolar transistor (HBT) structure for microwave power amplification.
2. Description of the Related Art
HBT's have been demonstrated to provide microwave power amplification more efficiently than metal-semiconductor field effect transistors (MESFETs) and high-electronmobility transistors (HEMTs). HBT microwave amplifiers offer high power density, sharp current cutoff, and negligible degradation in voltage breakdown characteristics when operated in class B or C high efficiency modes. The high power density of an HBT results in smaller device footprint, improved power combining efficiency, and smaller microwave monolithic integrated circuit (MMIC) chip size compared with a field effect transistor (FET) operating at the same power level.
The heterojunction nature of the HBT device structure is further compatible with material-composition-dependent self-termination (stop etch) wafer processing techniques. This, combined with the relatively coarse lithographic requirements (typically over 1.5 micron geometry compared with sub-0.5 micron geometry required for MESFETs operating at X-band frequencies and above), enable significant manufacturing yield improvement and cost reduction over that of conventional FET based MMICs.
The HBT features a heterojunction including a gallium arsenide (GaAs) base and wider bandgap aluminum gallium arsenide (AlGaAs) emitter which presents a potential barrier to hole flow from the base to emitter. This provides higher emitter efficiency than can be achieved in a conventional silicon based homojunction bipolar transistor structure. The base resistance can be reduced by heavy doping without sacrificing emitter efficiency. The frequency response of an HBT is higher than can be achieved with conventional bipolar transistors due to higher current gain and lower base resistance. However, the performance of HBTs operating at microwave frequencies is degraded by the base-collector capacitance of the device, especially in the grounded-emitter amplifier configuration. The large base-collector capacitance causes undesirable feedback and premature gain roll-off at elevated signal frequencies, limiting both amplifier efficiency and the upper limit of operating frequency.
A conventional HBT is described in an article entitled "Microwave Performances of NPN and PNP AlGaAs/GaAs Heterojunction Bipolar Transistors", by B. Bayraktaroglu et al, in 1988 IEEE MTT-S Digest, pp. 529-532, and has the structure illustrated in FIG. 1. The HBT as shown is an NPN device which is generally designated as 10, and includes a substrate 12 formed of GaAs or other semiinsulating material. A collector contact layer 14 is formed on a surface 16 of the substrate 12, preferably by ion implantation into the surface 16 as illustrated, or by epitaxy onto the surface 16 (not shown). For an NPN HBT, the collector contact layer 14 will be doped with an N-type impurity.
A collector layer 18 of GaAs doped with an N-type impurity is formed over the central portion of the collector contact layer 14. A base layer 20 of GaAs which is doped with a P-type impurity is formed over the collector layer 18. An emitter layer 22 of AlGaAs doped with an N-type impurity is formed over the central portion of the base layer 20.
The layers 18, 20 and 22 are deposited on the substrate 12 using molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD), or other suitable process, and subsequently etched to form the structure as illustrated. Collector ohmic contact metallizations or electrodes 24 are formed over the exposed laterally outer portions of the collector contact layer 14, preferably of gold/germanium (Au/Ge) or other electrically conductive metal or alloy. The heavily doped collector contact layer 14 and collector electrodes 24 provide electrical connection of a central active area 18a of the collector layer 18 to external devices (not shown). Base ohmic contact metallizations or electrodes 26 are similarly formed over the exposed laterally outer portions of the base layer 20, and an emitter ohmic contact metallization or electrode 28 is formed over the emitter layer 22.
The base-collector capacitance of the HBT 10 includes the depletion capacitance between the base layer 20 and collector layer 18, and the parasitic capacitance between the base layer 20 and collector contact layer 14. The depletion capacitance may be reduced by proton bombardment, prior to formation of the electrodes 24, 26 and 28, in areas 18b of the collector layer 18 laterally external of the emitter layer 22 to create a wider depletion layer.
However, the parasitic capacitance between the heavily doped base layer 20 and collector contact layer 14 in the areas 18b remains, and constitutes nearly 80% of the total base-collector capacitance in a typical HBT with 2 micrometer wide emitter fingers and one micrometer wide base contact layers. This undesired base-collector capacitance deleteriously limits the cutoff frequency and power gain of the HBT 10.
The base-collector capacitance may be reduced by using a "collector-up" HBT configuration as illustrated in FIG. 2. An HBT 30 includes a GaAs substrate 32, ion implanted or epitaxially grown AlGaAs emitter contact layer 34 formed on a surface 36 of the substrate 32, AlGaAs emitter layer 38, GaAs base layer 40, GaAs collector layer 42, emitter electrodes 44, base electrodes 46, and a collector electrode 48 as illustrated. The emitter layer 38 has a central active portion 38a, and outer portions 38b which are rendered semi-insulative by proton bombardment as described above. The HBT 30 differs from the HBT 10 in that the relative positions of the emitter and collector are reversed, with the collector layer 42 being above the emitter layer 38 in the HBT 30.
The base-collector capacitance is reduced in the HBT 30 due to the smaller width of the collector layer 42 compared to the collector layer 18 in the "emitter-up" HBT 10, thus improving the high frequency device characteristics. However, access to the thin base layer 40, which is typically less than 1,000 Angstroms thick, can only be accomplished by removing the overlying portions of the relatively thick collector layer 42. The manufacturing yield can be severely reduced by a small variation in the etch rate, or in the uniformity of the etch rate during the formation of the base electrodes 26. Incomplete removal of the collector material or over-etching of the base material will cause high base resistance and consequent degradation of the HBT performance characteristics.